The present invention relates to an electron beam exposure apparatus and its control method and, more particularly, to an electron beam exposure apparatus which irradiates an electron beam onto the object to be exposed such as a wafer placed on a stage and drawing a desired pattern on the object to be exposed, and its control method.
An electron beam exposure apparatus and its control method according to the present invention are suitably used for accurately detecting the position of a mark on a wafer applied with a resist, and accurately drawing a new pattern in correspondence with the position of the pattern already formed on the wafer.
An electron beam exposure apparatus draws a new pattern in correspondence with the position of the pattern already formed on a wafer. In order to attain such aligned drawing, the position of a position alignment mark formed on the wafer is detected in advance, and a new pattern is drawn with reference to the detected position.
FIG. 13 shows a conventional point beam type electron beam exposure apparatus using a spot-like beam. Reference numeral 201 denotes an electron gun for emitting an electron beam and ON/OFF-controlling emission of the electron beam in accordance with the pattern to be drawn; 202, a reduction electron optical system for projecting the electron beam coming from the electron gun onto a wafer 203 in a reduced scale with a point shape; 204, a deflector for scanning the point-like electron beam on the wafer; 205, an X-Y stage which moves while carrying the wafer; and 206, a detector for detecting reflected electrons and secondary electrons produced when a position alignment mark on the wafer is scanned with the electron beam.
With the above arrangement, conventionally, when a new pattern is drawn on a predetermined pattern region as one of a plurality of pattern regions already formed on the wafer, a linear position alignment mark of the predetermined pattern region formed on the wafer is located near a reference position of the point-like electron beam using the X-Y stage 206. Subsequently, the electron beam is scanned on the alignment mark by the deflector 204, and reflected electron and secondary electrons coming from the linear alignment mark at that time are detected by the detector 206. Based on the scanning position and the amount of reflected electrons/secondary electrons detected at that position, the position of the position alignment mark with respect to the reference position of the electron beam is determined. After that, the electron beam is scanned with reference to the determined position of the position alignment mark, and is ON/OFF-controlled in accordance with the pattern to be drawn, thus drawing a new pattern on the wafer.
FIG. 14A shows a plan view and sectional view of the position alignment mark on the wafer, FIG. 14B shows the scanning position when an electron beam at an acceleration voltage of 10 kV is used, and an alignment signal as the amount of reflected electrons/secondary electrons detected at that position, and FIG. 14C shows the scanning position when an electron beam at an acceleration voltage of 30 kV is used, and an alignment signal as the amount of reflected electrons/secondary electrons detected at that position.
As shown in FIG. 14B, when an electron beam at low acceleration voltage is used, the alignment signal has a low S/N ratio. This is because the electron beam is absorbed by a resist that covers the mark and reflected electrons/secondary electrons from the alignment mark become scarcer as the beam has a lower acceleration voltage. Hence, an electron beam at higher acceleration voltage is preferably used in position alignment.
However, upon drawing (by exposure) a pattern on a resist using an electron beam, when the irradiated electron beam repeats electron scattering in the resist and back scattering from a substrate, charges are accumulated on a portion around the irradiated position, and that portion is exposed. As the electron beam has higher acceleration voltage, since a broader region other than the irradiated position is exposed, the pattern to be drawn is distorted. Also, as the electron beam has higher acceleration voltage, the linearities of sensitivity of the resist and that for the irradiation amount worsen. Hence, an electron beam at low acceleration voltage is preferably used in pattern drawing (exposure).
Conventionally, the acceleration voltage is determined giving priority to one of position alignment and pattern drawing purposes.